1. Field of the Invention
This invention relates to a chemical vapor deposition method. More particularly, this invention relates to a process for the formation of a coating of phosphosilicate glass on a substrate by a chemical vapor deposition method (hereinafter, referred to as "CVD" or "CVD" method. This phosphosilicate glass (phosphorus-doped silicon dioxide, P-glass, or PSG) coatings obtained by CVD are widely used in the production of semiconductor devices such as IC, LSI and VLSI circuits. These PSG coatings act as an electrical insulating layer between metals within the device, as a passivation layer which protects the device from the environment, and as a gettering source, for example.
2. Description of the Related Art
A variety of deposited films or coatings such as polycrystalline silicon (or polysilicon), silicon nitride, silicon dioxide, and phosphorus-doped silicon dioxide (PSG) are widely used in the fabrication of semiconductor devices, and among these coatings, PSG coatings are more frequently used because of their excellent properties. Generally, the PSG coatings are deposited on the substrate by using various CVD methods, and the CVD methods and reactions can be carried out under many different conditions.
In the formation of the PSG coating by CVD, a gaseous mixture of monosilane (SiH.sub.4), phosphine (PH.sub.3), and oxygen (O.sub.2) is generally used as a reaction gas. The reaction gas is introduced into a reaction chamber to cause the following chemical reactions, thereby depositing the PSG coating: EQU SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2 EQU 4PH.sub.3 +50.sub.2 .fwdarw.2P.sub.2 O.sub.5 +6H.sub.2
The reactions can be carried out at atmospheric pressure or at a reduced pressure. The main advantage of these reactions is the low deposition temperature, lower than 500.degree. C., which allows the resulting PSG coatings to be deposited over aluminum metallization. This advantage will be appreciated when it is understood that, since the PSG coatings as the insulating layer or as the passivation or covering layer are deposited on the substrate or wafer after the formation of the aluminum circuitries thereon, it is necessary to carry out the reactions at temperatures lower than the melting point of aluminum, i.e., 660.degree. C., in order to avoid fusion of the aluminum circuitries. Generally, the above CVD reactions are referred to as a low-temperature CVD.
More particularly, hitherto, the PSG coating as an insulating layer has been deposited on the substrate or wafer surface by using a hot-wall, reduced-pressure CVD reactor, as shown in FIG. 1 for example. The CVD reactor of FIG. 1 has a reaction chamber 1 in the form of a quartz tube provided with two gas inlet tubes 2 and 3 and a gas exhaust tube 4. A gaseous mixture of SiH.sub.4 and PH.sub.3 is introduced through the inlet tube 2 into the reaction chamber 1, and oxygen gas is introduced therein through the inlet tube 3. Wafers 5 on which the PSG coating is deposited by CVD are stood vertically, perpendicular to the gas flow, in a basket 6. The reaction chamber 1 is heated with a heater 7 surrounding the chamber 1. Pressures in the chamber 1 are maintained in the range of from a reduced pressure of about 0.01 Torr to atmospheric pressure by controlling a reduced pressure exhaust system (not shown) connected with the exhaust tube 4. 0.2 Torr is maintained in the chamber 1.
The reaction gases are introduced into the reaction chamber at different flow rates. The flow rates of (SiH.sub.4 +PH.sub.3) mixed gas and O.sub.2 gas depend on various factors, such as the size of the chamber, number of wafers to be disposed in the chamber, and the like. The reaction gases are heated and reacted in the chamber while maintained at temperatures of from 300 to 450.degree. C.
An atmospheric-pressure CVD process also has been used for the formation of the PSG coating. The atmospheric-pressure CVD process can be carried out in the reactor shown in FIG. 2, for example. A reaction chamber 11 has a susceptor 9 on which a wafer 15 is positioned, and which is heated to temperatures of from 300.degree. C. to 450.degree. C. with the attached heater 10. A reaction gas consisting of SiH.sub.4, PH.sub.3 and O.sub.2 is introduced through a gas feed pipe 12 and a gas outlet 8 provided with orifices, in sequence, into the reaction chamber 11. After the CVD reaction is completed, the used gas is discharged through gas exhaust pipes 13 and 14.
The above-discussed reduced-pressure and atmospheric-pressure CVD processes can provide the PSG coatings having many advantages, such as a uniform and highly oriented coating, good electrical properties, and excellent resistance to the environment, but suffer from two serious problems. As is well known in the art, one problem is poor step coverage, and another is particles of the by-products produced during the CVD reactions, because the particles adhere to the wafer surface and thus wafers are unacceptable for use.
In the formation of the PSG coating by CVD, the following reactions are generated: EQU SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +H.sub.2 or SiO.sub.2 +H.sub.2 O EQU PH.sub.3 +O.sub.2 .fwdarw.P.sub.2 O.sub.5 +H.sub.2 or P.sub.2 O.sub.5 +H.sub.2 O
As a result, SiO.sub.2 is grown on the wafer, and P.sub.2 O.sub.5 is doped into the growing SiO.sub.2 layer to form the PSG coating. However, during these reactions, SiO.sub.2, P.sub.2 O.sub.5, H.sub.2 SiO.sub.3, and similar products (hereinafter referred to as by-products) are formed and deposited on the end portions or outlets of the gas inlet tubes, and these by-products give the following disadvantages.
Namely, referring now to FIG. 1, the problem of the particles of the by-products is caused in end portions of each of gas inlet tubes 2 and 3. Unavoidably, some of these by-products are formed and deposited in the end portions of the inlet tubes and the deposited by-products are sometimes dislodged and the particles thereof adhere as dust on the wafer surface. In addition, the by-products deposited on the outlets of the inlet tubes cause variations in the amount of gases introduced into the reaction chamber, thereby adversely affecting the uniform growth of the PSG coatings on the wafer.
Similar disadvantages also arise in the CVD reactor illustrated in FIG. 2. It was found that during the CVD reactions in the reactor, by-products such as SiO.sub.2, P.sub.2 O.sub.5 and H.sub.2 SiO.sub.3 are formed and deposited on the orifice-side surface of the gas outlet 8 and particles thereof are sometimes dislodged and adhere to the surface of the wafer 15 opposite the gas outlet 8. Experiments by the present inventors showed that about ten particles of the by-products, having a diameter of 1 .mu.m were adhered to a wafer having a diameter of 10 cm (4 inches).
In U.S. Pat. No. 4,625,678 issued Dec. 2, 1986, assigned to Fujitsu Limited, Shioya et al teach a plasma CVD apparatus for forming a deposited film such as silicide or metal film on a base body, which includes a gas feeding pipe and a heating device capable of heating at least a part of the gas feeding pipe inside the reaction chamber, thereby preventing the gas from condensing or solidifying. The heating of the gas feeding pipe as taught by Shioya et al can provide satisfactory results, and similar results cannot be obtained in the prior art CVD methods discussed above, because there are essential differences in the solidification or particle deposition mechanism of these CVD methods. In the prior art CVD methods discussed in the U.S. patent, a problem arose in that a reaction gas maintained at a high temperature is condensed and solidified immediately after introduction into a reaction chamber, due to a sudden reduction of the temperature thereof.